Lamperouge was his mother's maiden name. He and Nanaly reverted to it after the Britannian invasion to fake their deaths in the ensuing battles. One thing that is interesting to note, though, is that 'Lelouch Lamperouge' is most likely French in origin...and that 'Lelouch' sounds like a contraction of 'Le Louche', which means 'the suspicious one'. 'Lamperouge' means 'Red Lamp'.
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